Device for reading an elongate magnetic data carrier

ABSTRACT

A device for reading an elongate magnetic data carrier attached to or embedded in an article, having an active read head adapted to have a lateral extent greater than the width of the data carrier, preferably four times greater, such that the magnetic field produced by the read head results in a signal from the data carrier which has an amplitude which varies by less than 50% for head to data carrier spacings of from 0 to 60 microns. A wide read head can also make the reading of data more reproducible when reading narrow tracks using swipe readers, where precise alignment is difficult to achieve.

FIELD OF THE INVENTION

This invention relates to a device for reading an elongate magnetic datacarrier attached to or embedded in an article. It relates particularlyto active read heads of the type described in GB 2035659A forpermanently structured magnetic tape such as WATERMARK (Registered TradeMark), which is commercially obtainable from Thorn Secure ScienceLimited, Swindon, England.

BACKGROUND OF THE INVENTION

Active read heads of the above type provide an a/c energisingelectromagnetic field, and a detection circuit which is capable ofreading data on data carriers such as for example printed magneticpatterns or WATERMARK tape. However, a disadvantage of such active readheads is that the detected signal falls off rapidly with distance if thespacing between the data carrier and the read head is increased. FIG. 1shows experimental data indicating how the signal varies with spacing.This variation makes it difficult to read data when the spacing to thedata carrier is uncertain, or known to vary. For example, some banknotes have magnetic threads embedded in them, as disclosed in WO9819866A, the threads being exposed on the surface in some areas(windows) and buried under the surface in other areas. The present UK£20 note is an example of a banknote with a security thread embedded inthis fashion, so as to be exposed in spaced windows. Since a banknote isapproximately 100 microns thick, the variable spacing can causedetection problems when used with a read head which operates as shown inFIG. 1.

A second disadvantage of existing active read head systems for readingthe type of narrow magnetic thread used for banknotes arises from thevariable positioning of the threads. In the case of banknotes, thisvariability is introduced intentionally since perfectly aligned threadsmake it difficult for automated machinery to handle a stack ofbanknotes. For this reason a mechanised magnetic reader cannot readilypre-align the head with the thread, whilst a swipe reader may havedifficulty in tracking a buried thread, which is difficult to see. Knownactive read heads have a track width of typically 1.4 mm, which can bedifficult to align to a 2 mm wide thread. It is an object of the presentinvention to mitigate the above disadvantages.

SUMMARY OF THE INVENTION

According to the present invention there is provided a device forreading an elongate magnetic data carrier attached to or embedded in anarticle. The device comprises an active read head adapted to have atrack width having a lateral extent greater than the width of the datacarrier. The magnetic field produced by the read head in use results ina signal detected by the read head from said data carrier, which signalhas an amplitude which varies by less than 50% for head to data carrierspacings of from 0 to 60 microns. The signal can have an amplitude whichvaries by less than 25% for head to data carrier spacings of from 0 to60 microns. The present invention also relates to a swipe reader formagnetic data carriers (such as cards or documents) that include such adevice.

A passive read head having similarities to the present invention isdisclosed in U.S. Pat. No. 3,753,255. However, in this device the wideread head is simply employed to make overlap easier, as passive heads donot respond to a threshold signal level.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 shows a graph of the output voltage from a prior art read head asa function of spacing from a data carrier,

FIG. 2 shows a graph of the signal amplitude from a device according tothe invention as a function of spacing from a data carrier, and

FIG. 3 shows a read head and data carrier according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The experimental results displayed in FIG. 1 were obtained using a headtrack width of 1.4 mm and a data carrier consisting of a plastic cardhaving magnetic tape 10 mm wide affixed to it. The experimental resultsshown in FIG. 2 were obtained using a read head according to the presentinvention as shown in FIG. 3. The track width (1) of the read head (2)was 8 mm, the gap (3) between pole pieces (4,5) was 50 microns, and thepole pieces were made of mumetal. A head excitation voltage of 72 V peakto peak was used, and the data carrier (6) was a length of WATERMARKtape 1.8 mm in width, which was affixed to a paper substrate (7). Theexcitation voltage was an a/c waveform oscillating at several tens ofkHz.

The read head track width (1) is preferably greater than the datacarrier width (8) by at least the positional uncertainty of the thread,so that a mechanical reading system may be used without furtherpre-alignment whilst still ensuring that the read head overlies the datacarrier. Alternatively, it provides a greater positional tolerance for aswipe reader. Having a read head wider than the data carrier also makesthe coupling between the data carrier and the read head substantiallyconstant regardless of positional variation. This makes securityfeatures which rely on this property much more reliable. In addition,the fringing fields for a wide head are different to those for a narrowhead, resulting in a greater signal being produced when the head isspaced from the data carrier in the case of the present invention. Byincreasing the energising field from the active read head, greatervolumes of magnetic material will be magnetised at a greater distance.This compensates partly for the decrease in sensitivity with increasingspacing to the data carrier, resulting in the data shown in FIG. 2 inwhich the effective sensitivity of the read head is substantiallyconstant over several tens of microns variation in spacing to the datacarrier. With careful optimisation it is likely that the variation ofsensitivity with spacing shown in FIG. 2 can be improved upon. Thesignal shown in FIG. 2 has an amplitude which varies by less than 50%(in fact in the present example by less than 25%) for head to datacarrier spacings of from 0 to 60 microns.

The detection circuit for read heads according to the present inventionwill require slightly more gain than that employed for conventionalnarrow heads. This is because only a fraction of the wide head will beadjacent the narrow data carrier at any given time.

The active read head in the present invention is preferrably adapted tohave a track width having a lateral extent greater than twice (verypreferrably greater than 4 times) the width of the data carrier.

The elongate data carrier may be attached or embedded in a substrate,document, ticket, card, banknote, or other article.

Finally, the priority document (particularly the drawings) and theabstract accompanying the present description is incorporated herein byreference.

What is claimed is:
 1. In a system for reading an elongate magnetic datacarrier attached to or embedded in an article, including a read head anda data carrier, the read head being adapted to have a track width havinga lateral extent perpendicular to the direction of movement of the datacarrier greater than the width of the said data carrier perpendicular tothe direction of movement of the data carrier, the improvementcomprising the read head being constituted by an active read head havingmeans for providing an a/c energising magnetic field and a detectioncircuit arranged so that the active read head including the means forproviding the a/c energising magnetic field and the detection circuitcan be located on the same side of the data carrier, the lateral extentof the active read head being at least twice the lateral width of thedata carrier, thereby producing a magnetic field which results in use ina signal, detected by said read head from said data carrier, having anamplitude which varies by less than 50% for head to data carrierspacings of from 0 to 60 microns.
 2. In a system as claimed in claim 1,wherein the lateral extent of the active read head relative to thelateral extent of the data carrier being such that the signal detectedby the active read head has an amplitude which varies by less than 25%for head to data carrier spacings of from 0 to 60 microns.
 3. In asystem as claimed in claim 1 wherein the lateral extent of the of theactive read head is greater than four times the lateral width of thedata carrier.
 4. A swipe reader for magnetic data carriers comprisingthe system of claim 1.